Mechanical characterization of materials has to contend with challenges such as non-destructive measurements (in the case of expensive or rare sample materials, or to allow the re-use of the same sample after mechanical characterisation), hyper-frequency viscoelastic characterization (wide-band frequency viscoelasticity, elasticity or viscosity), viscoelastic evaluation as function of temperature, fast measurements (important when mechanical parameters or environmental configuration change rapidly) and simplicity of use. Precise, accurate and fast measurement of viscoelastic spectroscopy as function of temperature is essential for the conception, the optimization, the quality control, and the safe and efficient use of material in real conditions.
For example, the seals, gaskets or o-rings installed in major parts of a jet fighter have to be in accordance with critical thermo-mechanical specifications since, during the takeoff and the flight, the material is subjected to a huge increase or decrease in temperature in a very short time and a significant vibration in a high frequency band. Another example is the rubber employed to build tires of airplanes for which they has to be optimally designed and tested to withstand significant compression, very fast temperature increase and wide-band high frequency vibration during plane landing. Another example, in the biomedical application, is the development of implantable synthetic heart tissues that have to mimic as closely as possible the mechanical properties of real human heart tissue. Since the heart is a moving and vibrating tissue, accurate knowledge of its viscoelasticity is a key point parameter that will have an important impact on it safety in real conditions (i.e. implanted in humans). Such measurement also has to be done non-destructively and contactless for quality control of biomaterials before implantation.
Various systems and methods have been contemplated in the prior art to measure viscoelastic properties of material samples. Rotational rheometers (RHE), Dynamic Mechanical Rheological Testing (DMRT), Dynamic Mechanical Analysis (DMA) of materials, indentation system (IS) are typical systems currently in use to that purpose.
RHE and DMRT instruments are based on the characterization of a disk-shaped sample sandwiched between two plates, the upper plate applying a shear strain to the tested sample and measuring in the same time the oscillatory shear stress. The DMA instruments are based on the compression of cylindrical or rectangular samples using a large rigid plate, the opposite plate being connected to a load sensor employed to measure the induced stress. In the case of IS, the sample is placed on a rigid plate connected to a load sensor and a small tip of different size and shape is used to indent, at different depth, the upper surface of the sample.
All the above-mentioned systems are mainly limited in performance by their relatively low test frequency range (typically below 200 Hz), the long measurement time (typically 30 min for a full frequency sweep), the restriction of sample size and shape (thin disk or slice usually difficult to prepare), the complex fixtures used to enable recording of the sample behaviour and the use of functional mechanical elements in contact with the tested sample (which results in the limitation to one sample measurement at a time and in the destruction of tested samples).
United-States patent application No 2010/012092 by Cloutier et al. discloses a system and method for detection, characterisation and imaging of a heterogeneity using shear wave induced resonance. In Cloutier et al, the viscoelastic sample is contained as a heterogeneity within a non-rigid, viscoelastic medium. This latter can be contained in a rigid container or simply in contact with it. In an embodiment, a system of particular interest is taught comprising a vibration source; a container for a sample, said container being connected to the vibration source; a vibration detector; and a processor, wherein the vibration source generates shear waves that induce vibrations and the resonance of the sample in said container, the vibration sensor measuring the sample vibrations and resonances and the processor determining the viscoelasticity of said sample from said resonances. However, the disclosure mainly aims at detecting and characterizing a heterogeneity in a body and provides very limited hints for investigating and developing possible applications in the field of materials testing.
Therefore, it is desired to overcome or reduce at least some of the above-described problems and limitations of the prior art.